Microsensors for Automotive Applications - Metrology and Test
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Microsensors for Automotive Applications – Metrology and Test Gottfried Flik, Heinz Eisenschmid, Carsten Raudzis, Frank Schatz, Winfried Schoenenborn1 and Hans-Peter Trah Corporate Research, Robert Bosch GmbH, D 70049 Stuttgart, Germany 1 Automotive Electronics Division, Robert Bosch GmbH, D 72762 Reutlingen, Germany
ABSTRACT According to market surveys automotive microsensors will evolve into a multi-billion dollar business by 2005. Key roles are attributed to inertial sensors for passenger safety systems, and mass flow and pressure sensors for engine management systems. Thin film techniques together with silicon bulk or surface micromachining have been established as preferential processes to achieve reduction of sensor size, weight and cost along with improvements of sensor functionality and reliability. Enhanced sensor performance often pushes the limits of process technology and therefore the need arises very early in the MEMS design process to identify materials and geometry related parameters which are critical with respect to their tolerance band specifications. In order to control these critical parameters, automated wafer level test procedures need to be developed (based preferentially on electrical quantities) and additionally considered for in the sensor design phase (design for test). In analogy to microelectronics 2D wafer maps of critical parameters may give hints on how to improve process stability and how to adapt the sensor design in order to optimize yield. Examples of critical model parameter variations include thermal conductivity, thickness, and shear modulus of thin films.
INTRODUCTION – AUTOMOTIVE SENSOR MARKET Sensors are enabling components for the functionality of an ever increasing number of electronic systems in the car. Especially for engine emission control and passenger safety systems, microsensors have taken over a large share of the market volume and are beginning to penetrate into the convenience equipment sector. Among these acceleration sensors, gyros, pressure and mass flow sensors are manufactured today efficiently in very large volumes taking advantage of MEMS batch process technologies. Figure 1 shows examples of MEMS transducer elements (dies) and their corresponding packages. It is believed [1] that automotive microsensors in general will take a market share of 30% by 2005 corresponding to a market volume of almost 3 billion dollars. However, requirements to be met in the automotive market are severe. Demands for increased sensor performance and tightened functional tolerance specifications (sensitivities, resolution, selftest capability, EMI, overvoltage, reverse polarity and EMI protection, …) have to be met, aiming at reduced cross sensitivities, time constants, noise levels, reduced weight, size and power consumption simultaneously. In addition reliability of sensor performance has often to be guaranteed within 1% over the lifetime of an automobile, and thus provisions have to be made for the sensor system to B1.1.1
Figure 1. Examples of automotive microsensors. wit
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